As with the other CTPs, you have to uninstall any previous Team System
for Database Professionals install before installing CTP 5. When you
run the downloaded .exe, it'll tell you that and then prompt you for a
location to extract the .exe file to.

If you have projects from previous versions, they won't work with this latest CTP. From the Readme.doc that extracts:

2.2.2. Compatibility with prior CTPs

If you have database projects that you created using a prior CTP of
Team Edition for Database Professionals, you cannot open them using
CTP5. To implement new features, some breaking changes were required in
the file formats for database projects, data generation plans, and
database unit tests.

Also, it will install a default instance of SQL Server
Express if you don't already have one installed. There isn't an option
in the installer to not install it.